INTRACELLULAR CALCIUM-CONCENTRATION AND ACTIVATION OF THE NA+ H+ EXCHANGER IN ESSENTIAL-HYPERTENSION/

To investigate the relationship between changes in intracellular calci um concentration ([Ca2+](i)) and agonist-induced activation of the Na/H+ exchanger in essential hypertension (EH), platelet [Ca2+](i) and p H(i) were measured in 24 patients with EH (14 males) aged 48 +/- 2 yea rs and 23 matched normotensive controls (NT) (12 males) aged 45 +/- 3 years. Measurements were done with spectrofluorimetry using the dyes F ura-2 for [Ca2+](i) and BCECF for pH(i). [Ca2+](i) and pH(i) were meas ured in the resting condition and after stimulation in vitro with 0.1 U/ml human thrombin. The thrombin-induced rise in pH(i) was Na+ depend ent and amiloride sensitive, indicating that it was mediated by the Na +/H+ exchanger. Unstimulated [Ca2+](i) was higher in patients with EH than in NT (60 +/- 3 vs. 48 +/- 1 nmol/liter, P < 0.005), but there we re no differences in resting pH(i) between both groups (7.16 +/- 0.01 vs. 7.16 +/- 0.008). In the presence of 1 mmol/liter external calcium (Ca-o(2+)), thrombin-induced increment in [Ca2+](i) was significantly greater in patients with EH than in NT (281 +/- 21 vs. 206 +/- 19; P < 0.05) as was the pH(i) increment (EH: 0.137 +/- 0.01; NT: 0.095 +/- 0 .01; P < 0.05). Both agonist-induced increments in [Ca2+](i) and in pH (i) were correlated with mean arterial pressure (MAP) only in the EH g roup (r = 0.58, P < 0.005 and r = 0.59, P < 0.005, respectively). The agonist-induced rise in pH, was positively correlated with the rise in [Ca2+](i) both in the EH group (r = 0.65, P < 0.001) and in the NT (r = 0.55, P < 0.01). At higher doses of thrombin (2.5 U/ml), the enhanc ed increment in both platelet [Ca2+](i) (EH: 732 +/- 20; NT: 619 +/- 1 5 nmol/liter, P < 0.05, N = 6) and in pH(i) (EH: 0.193 +/- 0.001; NT: 0.150 +/- 0.004 P < 0.005, N = 6) was also observed in patients with E H. In the absence of Ca2+(o) (chelated with EGTA), there were no signi ficant differences between patients with EH and NT in thrombin-induced increases neither in [Ca2+](i) (EH: 140 +/- 18; NT: 121 +/- 22 nmol/l iter) nor in pH(i) (EH: 0.098 +/- 0.01; NT: 0.079 +/- 0.01). When the agonist-induced increase in [Ca2+](i) was prevented with the Ca-i(2+) chelator MAPTAM, no rise in pH(i) was observed in either group. In con clusion, in platelets challenged with thrombin, the agonist-induced ac tivation of the Na+/H+ exchanger occurs in correlation with the increm ent in [Ca2+](i), and both are significantly increased in patients wit h EH only in the presence of Ca-o(2+). Furthermore, the magnitude of t he agonist-induced changes in [Ca2+](i) and in pH(i) showed a direct r elationship with MAP in the EH group. The agonist-induced activation o f the Na+/H+ exchanger requires a rise in [Ca2+](i) both in patients w ith EH and in NT subjects.